Impregnated cathodes



Aug. 8, 1961 J. H. APELBAUM El AL IMPREGNATED CATHODES Filed Feb. 2-1,

INVENTORS JOSEPH H. APLEBAUM KENNETH Inl- DUDLEY ATTORNE I United StatesPatent 2,995,674 IMPREGNATED CATHODES Joseph H. Apelbaum, Waban, andKenneth W. Dudley,

South Sudbury, Mass., assignors to Raytheon Company, a corporation ofDelaware Filed Feb. 27, 1959, Ser. No. 796,109 12 Claims. (Cl. 313-346)This invention relates to a cathode for electron discharge devices, andmore specifically, to a tungsten cathode impregnated with a mixture ofone or more alkaline earth oxides and chromic oxide.

An object of this invention is to produce an impregnated cathode whichis characterized by relatively high thermionic emission and which iscapable of withstanding relatively high operating temperatures of theorder of 1100 C.

In accordance with this invention, a sintered porous tungsten body ormatrix is impregnated with an electronemissive impregnant made of afused mixture of chromic oxide and one or more of the group known' asalkaline earth oxides, including barium oxide. The impregnant preferablyconsists of barium oxide and chromic oxide, although satisfactoryoperation has been obtained when either calcium oxide or strontium oxideis added to the barium oxide. Such cathodes, unlike many other oxidecathodes, are capable of satisfactory operation at temperatures of theorder of 1100" C. with reasonably high electron emission densities.

The cathodes according to the invention are made by firing a tungstencompact at a high temperature until the compact attains sufiicientstrength and the desired porosity. A suspension of chromic oxide and oneor more alkaline earth oxides-one of which is barium oxide-is applied tothe surface of the fired tungsten compact. The coated tungsten compactis next fired to a temperature well above the melting point of theimpregnating compound, until the latter becomes dispersed substantiallyuniformly throughout the porous mass of the tungsten body. Duringoperation of the electron discharge device in which the cathode isplaced, a reaction occurs between the impregnant and the tungsten bodysuch that some free barium is formed which diffuses out onto the surfaceof the tungsten body in a very thin layerprefera'bly less than onemolecular layer thickat a rate determined by such factors as theporosity of the tungsten body, cathode operating temperature, and thecomposition of the impregnant. The chromic oxide has been found to slowdown the rate of formation of the barium and thereby extends the life ofthe cathode. The electron-emissive surface is replenished by gradualdiffusion of the electronemitting material toward the surface of thetungsten body. In those cases where alkaline earth oxides, such ascalcium oxide and strontium oxide, are added to the barium oxide, acomposite emissive layer is formed at the surface of the tungsten bodywhich, like the emissive layer formed with barium oxide and chromicoxide alone, is replenished at a rate dependent upon the factorsabovementioned.

The single figure of the drawing shows in cross section one form ofcathode assembly embodying the present invention.

The cathode assembly includes a metal supporting sleeve 12 whichpreferably is made of a refractory metal, such as molybdenum ortungsten, capable of withstanding relatively high cathode operatingtemperatures. End shield members 13 and 14 may be attached to thesupporting sleeve 12 adjacent both ends thereof to prevent emission ofthe cathode material in a direction parallel to the longitudinal axis ofthe cathode assembly. An elongated heater coil 15 is included within thecentral bore of supporting sleeve 12. One end of heater coil 15 facethereof.

may be attached to an electrically conductive member 16, which, in turn,is secured to the inner periphery of the supporting sleeve 12 at one endthereof. The other end of the heater 15 is attached to a heater lead-inconductor 18 which passes through the central aperture in the end shieldmember 14 and then external to the evacuated envelope of the electrondischarge device, not shown, in which the cathode assembly 10 is to bemounted. For a description of a typical electron discharge device of themagnetron type in which a cathode according to the invention may beincorporated, attention is directed to the United States Patent No.2,542,908 of Palmer P. Derby, issued February 20, 1951. Theelectron-emissive portion of the cathode assembly 10 includes a tubularcathode body 19 of porous tungsten whose surface, during actual tubeoperation, is coated with a very thin layer 20 of emissive materialdiffused from the pores of the tungsten body 19. In practice, this layeris slightly less than one molecular layer in thickness and is greatlyexaggerated in the drawing for the sake of clarity.

A mass of tungsten powder is compressed in a mold to produce a tungstencompact which conforms generally to the desired shape and size of thefinished cathode body. The tungsten compact should be somewhat oversize,since subsequent firing steps cause some shrinkage. In one instance, thetungsten powder was subjected to a pressure of fourteen tons per squareinch. This pressure will depend partly on the size of the tungstenparticles and upon the desired porosity of the tungsten compact.

The tungsten compact next is fired at an elevated temperature in afurnace containing a vacuum or non-oxidizing atmosphere, such as heliumor hydrogen.

It has been found that firing at a temperature of approximately 2350" C.for about twenty minutes produces a cathode of the necessary structuralstrength and porosity. The strength, as well as the porosity of thetungsten body is a function of both firing time and firing temperature.If the firing temperature is reduced, the firing time must be increased,and vice versa. The porosity of the tungsten compact partiallydetermines the diffusion rate, that is, the rate at which the emissivematerial diffuses out of the pores of the tungsten body onto the sur- Ifthe tungsten body is too porous, the emissive material will escape toorapidly from the tungsten body and the cathode life will be limited.Similarly. if the tungsten body is too dense, the emissive material willnot diffuse out to the surface rapidly enough to provide properemission. Moreover, the porosity will determine the amount of impregnantthat can be made to penetrate initially into the tungsten body; thehigher the porosity of the tungsten body, the greater will be theinitial supply of emissive material available in the tungsten body. Fora given cathode operating temperature, an equilibrium condition isdesired such that optimum coverage of the surface of the tungsten bodyis attained, since the surface coverage depends upon the amount ofelectron-emissive material which diffuses out of the tungsten pores ontothe surface of the tungsten body. This optimum surface coverage isusually slightly less than a monomolecular layer of theelectron-emissive material. If the diffusion rate is lowered, because ofdecreased porosity of the tungsten body, it is necessary to resort tohigher cathode operating temperatures to produce the same emissioncurrent density.

A mixture including barium oxide (BaO) and chromic oxide (Cr O suspendedin a suitable binder such as methyl alcohol which evaporates readilyduring the firing process, is applied to one or both surfaces of thetubular tungsten body 19, as by painting or spraying. The coatedtungsten body 19 is then fired at a temperature well in excess of themelting point of the comminuted mixture in a furnace containing a vacuumor a nonoxidizing atmosphere, such as hydrogen.

In a preferred embodiment of the invention, the impregnant compositionincluded three parts of barium oxide by weight to one part chromic oxideby weight. Another impregnant composition may include four parts ofbarium oxide by weight to one part of chromic oxide by weight. Anothercomposition may include approximately 70 percent barium oxide by weight,18 percent chromic oxide by weight and 12 percent strontium oxide byweight, corresponding substantially to four moles-of BaO, one mole of CrO and one mole of SrO. Still another composition may consist ofapproximately 75 percent barium oxide by weight, 18 percent chromicoxide by weight, and 7 percent calcium oxide by weight, correspondingsubstantially-to four moles of BaO, one mole of Cr O and one mole ofCaO. The impregnant-coated tungsten body may be fired at a temperatureof the order of 1800 C. for about one minute. The firing time must notbe too long lest excessive emitting material evaporate out of thetungsten matrix during firing. During the firing process, the impregnantmelts and flows into the pores of the tungsten body by capillary action.The impregnated cathode, after cooling, is now ready for insertion in anelectron discharge device, such as a magnetron. When the cathode heateris energized such that the cathode operating temperature is of theproper value, the impregnant reacts with the tungsten in the cathodebody to form barium (in the case of barium oxide and chromic oxide) orbarium and either strontium or calcium (when the impregnant containseither strontium oxide or calcium oxide in addition to the bariumoxide).

This invention is not limited to the particular details of construction,materials and processes described, as many equivalents will suggestthemselves to those skilled in the art. It is accordingly desired thatthe appended claims be given a broad interpretation commensurate withthe scope of the invention within the art.

What is claimed is:

'1. A cathode comprising a porous body of tungsten and a mixturecomprising chromic oxide and an alkaline earth oxide disposed withinsaid body.

2. A cathode comprising a body of tungsten of porous construction and asupply of electron-emissive material disposed within said porous bodywhich comprises a mixture of chromic oxide and an alkaline earth oxideincluding barium oxide.

3. A cathode comprising a porous body of tungsten and a mixturecomprising chromic oxide and barium oxide disposed within said body.

4. A cathode comprising a porous sintered body of tungsten and a supplyof electron-emissive material disposed within said porous body whichcomprises one part 4 of chromic oxide by weight and three parts ofbarium oxide by weight.

5. A cathode comprising a porous sintered body of tungsten and a supplyof electron-emissive material disposed within said porous body whichcomprises a mixture ofone part of chromic oxide by weight and four partsof barium oxide by weight.

6. A cathode comprising a body of tungsten of porous construction and asupply of electron-emissive material disposed within said porous bodywhich comprises a mixture of chromic oxide, barium oxide, and calciumoxide.

7. A cathode comprising a body of tungsten of porous construction and asupply of electron-emissive material disposed within said porous bodywhich comprises a mixture of chromic oxide, barium oxide, and strontiumoxide.

8. A cathode comprising a porous sintered body of a refractory metal anda mixture containing chromic oxide and an alkaline earth oxide includingbarium oxide disposed Within the pores of said body.

9. A cathode comprising a porous sintered body of refractory metaltungsten and a supply of electron-emissive material disposed within saidporous body which comprises a mixture of approximately 18 percent ofchromic oxide by weight, approximately 75 percent of barium oxide byweight, and approximately 7 percent of calcium oxide by weight.

10. A cathode comprising a porous sintered body of refractory metaltungsten and a supply of electron-emissive material disposed within saidporous body which comprises a mixture of approximately 18 percent ofchromic oxide by weight, approximately percent of barium oxide byweight, and approximately 12 percent of strontium oxide by weight.

11. A cathode comprising a porous sintered body of refractory metaltungsten and a supply of electron-emissive material disposed within saidporous body which comprises a mixture of approximately 18 percent ofchromic oxide by weight, and the remainder of at least one alkalineearth oxide.

12. A cathode comprising a porous sintered body of refractory metaltungsten and a supply of electron-emissive material disposed within saidporous body which comprises a mixture of approximately 18 percent ofchromic oxide by weight, and the remainder of barium oxide.

References Cited in the file of this patent UNITED STATES PATENTS2,488,727 Klasens et al. Nov. 22, 1949 2,700,000 Levi Jan. 18, 1955FOREIGN PATENTS 185,472 Austria May 11, 1956 194,994 Austria Jan. 25,1958

1. A CATHODE COMPRISING A POROUS BODY OF TUNGSTEN AND A MIXTURECOMPRISING CHROMIC OXIDE AND AN ALKALINE EARTH OXIDE DISPOSED WITHINSAID BODY.